(a) Field of the Invention
The present invention relates to a deflection yoke for a cathode ray tube, and more particularly, to a ferrite core of a deflection yoke used in a cathode ray tube.
(b) Description of the Related Art
A typical cathode ray tube is structured with an electron gun mounted within a neck, a shadow mask and a phosphor screen mounted to a panel, and a deflection yoke mounted to an outer circumference of a funnel. Electron beams emitted from the electron gun are deflected by a magnetic field generated by the deflection yoke, and the deflected electron beams pass through the shadow mask to land on the phosphor screen and illuminate the screen. Predetermined images are realized through this process.
The deflection yoke includes a horizontal deflection coil and a vertical deflection coil. The two coils are mounted to the outer circumference of the funnel in a state adjacent to one another. Further, a core (typically made of ferrite) is provided covering the vertical deflection coil. A horizontal deflection current flows through the horizontal deflection coil to generate a horizontal deflection magnetic field, and a vertical deflection current flows through the vertical deflection coil to generate a vertical deflection magnetic field.
The electron beams emitted from the electron gun progress toward the phosphor screen by an anode voltage (i.e., by attraction to the positive voltage) to enter a region where there is a deflection magnetic field generated by the deflection yoke. While in the deflection magnetic field, the electron beams receive a force according to Fleming's left hand rule to be deflected by a deflection current. The electron beams then scan the phosphor screen to realize predetermined images.
The power consumed to deflect the electron beams is indicated by a flux density B generated by the vertical deflection coil and the horizontal deflection coil. Flux density B is given by Equation 1 as follows.B=4π*10−7(ni/Dy)  [Equation 1]
where n is the number of windings of the deflection coils, i is the deflection current (in units of amperes), and Dy is an inner diameter of the ferrite core (given in units of centimeters).
Therefore, the power consumed by the deflection yoke depends, to a great extent, on the size of the inner diameter of the ferrite core. That is, power consumption may be best reduced by reducing the inner diameter of the ferrite core. Accordingly, energy-saving cathode ray tubes are now being developed, in which the shape of the funnel of the CRT is changed from having a cylindrical cross section to approximately a rectangular cross section and the inner diameter size of the deflection yoke is reduced.
If cross sections of the area where the deflection yoke is mounted are compared between the above energy-saving CRT and a traditional CRT, the traditional CRT forms a circle in this area while the energy-saving CRT has a cross section that is approximately rectangular, that is, a cross section with circular arcs connected at four corners that are substantially at right angles. With this configuration, the energy-saving CRT has an inner diameter that is reduced by 30% in the horizontal axis direction when compared to the traditional CRT.
Accordingly, the horizontal coil, vertical coil, and ferrite core of the deflection yoke are also reduced in size. Since the horizontal coil and vertical coil are structured by bending copper wire that is coated with flexible enamel resin in a saddle shape, these elements may be formed through a shrinking process. However, the ferrite core is formed by pressing iron oxide containing iron, zinc, manganese, copper, nickel, barium, yttrium, etc. in a mold, then sintering the resulting material in a furnace at a temperature of approximately 1,400° C. If the sintered material is used as is, an error results in the precision of its dimensions of roughly ±0.5 mm. Therefore, grinding of the material is performed to more precisely form the ferrite core.
The ferrite core produced in this manner includes a portion formed as a circle, and portions that are substantially rectangular and formed of three circles of differing radii. About 75% of this configuration is approximately rectangular.
In an effort to increase productivity, grinding of the ferrite core is performed by contacting a rotating grindstone to an inner surface of the core and performing grinding to a depth of approximately 0.5 mm, with grinding being discontinued when within ±0.1 mm of the desired depth to thereby ensure precision in the final dimensions. That is, the plate core is fixed to a grinding jig, and the grinding jig is rotated at a low speed (roughly 300 rpm), and the grindstone is rotated in a direction opposite to the rotating direction of the grinding jig at a relatively high speed (roughly 900 rpm) to thereby perform grinding of the ferrite core.
With the above grinding method, grinding may be performed on areas formed as a circle, but is not possible on the approximately rectangular areas, which comprise 75% of the core as described above. As a result, only the remaining 25% of the core undergoes grinding while the majority does not, thereby resulting in problems with respect to the precision in the dimensions of the core. This affects the operation of the deflection yoke such that the overall accuracy of the CRT is negatively affected.